1. Field of the Invention
The invention relates to oxidation of carbon nanotubes. Specifically, the invention relates to photooxidation of carbon nanotubes, such as single-walled carbon nanotubes.
2. Background of the Disclosure
A fullerene is any molecule composed entirely of carbon. Fullerenes are frequently in the form of a hollow sphere, cylinder, ellipsoid or a tube. Cylindrical fullerenes are called carbon nanotubes. Since the discovery of carbon nanotubes and later single-wall carbon nanotubes, research has been conducted to exploit their unique mechanical, electrical, and thermal properties to create multifunctional composite materials. One such property is that carbon nanotubes have a substantially longer length than diameter. Specifically, single-wall nanotubes (“SWCNTs”) have a length that can be a million times larger than its diameter. In addition, SWCNTs may have a tenfold higher tensile strength and a fivefold higher Young's modulus than conventional carbon fibers. Actually, research has shown that single-wall carbon nanotubes have the highest conductivity of any known fiber, a higher thermal conductivity than diamond, and the highest stiffness of any known fiber.
As a result of these exceptional properties, the addition of carbon nanotubes into polymers can significantly increase mechanical, electrical and thermal properties. Clearly, use of carbon nanotubes as primary or secondary reinforcements in polymers or ceramics could lead to new materials with significantly enhanced mechanical strength and electrical and thermal conductivity. Use of carbon-nanotube-reinforced materials in aerospace components, for example, will enable substantial reductions in component weight and improvements in durability and safety. Potential applications for single wall carbon nanotubes include lightweight components for vehicle structures and propulsion systems, fuel cell components (bipolar plates and electrodes) and battery electrodes, and ultra-lightweight materials for use in solar sails.
However, dispersion of carbon nanotubes, such as a SWCNTs, in a given polymer matrix can be problematic. Pure SWCNTs tend to “rope up” due to strong Van der Waals attractive forces and, as a result, are not readily dispersed in organic solvents. Instead, SWCNTs tend to agglomerate in a polymer matrix. In addition, strong interfacial bonding between the SWCNTs and the polymer matrix is essential to deriving materials with enhanced electrical, thermal and mechanical properties.
Accordingly, the key to the use of carbon nanotubes as polymer additives is to develop the proper functionalization chemistry that enables dispersion of carbon nanotubes into the polymer matrix without adversely affecting the chemical structure and beneficial properties of the carbon nanotubes. Specifically, functionalizing carbon nanotubes enhances their ability to be incorporated into polymer matrixes and enhances their bonding with the matrix. A variety of chemistries have been developed to functionalize carbon nanotubes. One approach to carbon nanotube functionalization is to oxidize the end and side wall carbons, typically by treatment with a strong oxidizing agent or refluxing in strong acids. However, this approach can damage the tubes, leading to the introduction of defects. Specifically, defects can be introduced into the side-walls of the nanotubes, changing the carbon-carbon bonds from sp2 to sp3 hybridization, and reducing the electrical and thermal conductivities. In addition, strong oxidizing agents or refluxing in strong acids can cause the carbon nanotubes to be “cut” to smaller lengths. Reducing length of the carbon nanotubes by cutting reduces their aspect ratio and negatively impacts their mechanical properties and efficacy as a reinforcement in polymers. In addition, the chemicals used to oxidize the SWCNT are hazardous and have a negative impact on the environment. An approach to functionalizing carbon nanotubes, such as SWCNTs, which maintains their length to diameter ratio and/or reduces environmental impact, is desirable.